1. Field of the Invention
The present invention relates generally to frequency synthesizer-type television signal receiving apparatus and controlling methods therefor and, more particularly, to a frequency synthesizer-type television signal receiving apparatus for use in a television set, video cassette recorder (VCR) or the like, which utilizes an AFT (Automatic Fine Tuning) signal detecting circuit for channel selection, and controlling method of such AFT signal detecting circuit.
2. Description of the Background Art
Voltage synthesizer system and PLL frequency synthesizer system are well known as a television signal receiving apparatus for use in a conventional television set or a VCR, which converts a signal of a desired channel among received RF television signals into an intermediate frequency (IF) signal by means of a tuner.
FIG. 1 is a schematic block diagram showing a conventional television signal receiving apparatus of such a voltage synthesizer system as described above. Referring to FIG. 1, the RF television signals received by an antenna 1 is applied a tuner 2 mainly comprising a mixing circuit 2a and a local oscillating circuit 2b. The RF television signals applied to tuner 2 are mixed in mixing circuit 2a with the local oscillating signal from local oscillating circuit 2b. The local oscillating frequency of local oscillating circuit 2b is controlled by a control voltage applied from a control circuit 3. Control circuit 3 changes its control voltage in response to a control signal generated from a channel selection control circuit 4 based on channel selection operation by a user. Namely, a local oscillating output of the local oscillating frequency predetermined so as to correspond to a channel desired by the user is mixed with the RF television signal of that channel in mixing circuit 2a and the mixture is extracted as an IF signal.
In this way, the IF signal extracted from tuner 2, after having the band thereof limited by a SAW filter 5, is applied to an intermediate frequency processing circuit 6 comprising ICs. Intermediate frequency processing circuit 6 mainly comprises a video detecting circuit 6a and an AFT signal detecting circuit 6b. The IF signal applied to intermediate frequency processing circuit 6 is video-detected by the well known video detecting circuit 6a and the video-detected signal is supplied as a color video signal through a terminal 7. The supplied color video signal is applied to a picture tube (not shown) through a video amplifying circuit (not shown), for example.
It sometimes happens that the local oscillating frequency of local oscillating circuit 2b in tuner 2 fluctuates due to temperature drift or the like and consequently the actual local oscillating frequency deviates from the correct oscillating frequency predetermined for each channel. Such deviation of the local oscillating frequency causes a deviation of the IF signal frequency of the desired channel, resulting in deterioration of quality of the reproduced picture.
As a method of compensating such deviation in a local oscillating frequency, widely known is feed-back control using a so-called AFT loop. More specifically, referring to FIG. 1, the IF signal applied to intermediate frequency processing circuit 6 is applied to both video detecting circuit 6a and AFT signal detecting circuit 6b. As will be described later in detail, the AFT signal detecting circuit 6b comprises a double-balanced differential amplifier to generate an AFT signal having a signal voltage corresponding to such fluctuation in the IF signal frequency as described above and feed back the same to control circuit 3. In response to the AFT signal, control circuit 3 adjusts the control voltage to be applied to local oscillating circuit 2b to correct the local oscillating frequency of local oscillating circuit 2b so as to compensate the fluctuation in the IF signal frequency.
Meanwhile, in the television signal receiving apparatus shown in FIG. 1, AFT signal detecting circuit 6b might generate an erroneous AFT signal and apply the same to control circuit 3 due to unstable operation of the entire receiving apparatus immediately after the channel selection, that is, channel switching. Thus, the conventional voltage synthesizer system television signal receiving apparatus is structured such that an AFT signal detecting circuit of FIG. 1 is rendered non-operative (muted) during and before and after the channel selection.
FIG. 2 is a schematic block diagram showing the conventional television signal receiving apparatus of a PLL frequency synthesizer system. The television signal receiving apparatus shown in FIG. 2 is the same as that shown in FIG. 1 except the following points.
Namely, control circuit 3 comprises a 1/n frequency dividing circuit 3a, a reference signal generating circuit 3b and a phase comparing circuit 3c. The local oscillating signal from local oscillating circuit 2b is applied to frequency dividing circuit 3a wherein the signal frequency is divided by n, so that the frequency divided signal is applied to one input of phase comparing circuit 3c. The other input of phase comparing circuit 3c receives a reference signal of a fixed frequency from reference signal generating circuit 3b. Phase comparing circuit 3c compares the phases of both inputs to output an error signal to be applied as a control voltage to local oscillating circuit 2b of tuner 2. Namely, dividing circuit 3a, phase comparing circuit 3c and local oscillating circuit 2b constitute a phase locked loop (PLL). When selecting channels, channel selection control circuit 4 comprising a microcomputer changes the frequency dividing ratio 1/n of frequency dividing circuit 3a in accordance with the channel to be selected. As a result, the control voltage outputted from phase comparing circuit 3c is changed, so that the local oscillating frequency of local oscillating circuit 2b is also changed to carry out channel selection.
In principle, the PLL frequency synthesizer system television signal receiving apparatus shown in FIG. 2 does not require a provision of such AFT signal detecting circuit as shown in FIG. 1 because of highly precise control of the local oscillating frequency by using the microcomputer.
In a specific case, however, an AFT signal detecting circuit is provided in the PLL frequency synthesizer system television signal receiving apparatus. For example, in receiving apparatus capable of receiving such cable broadcasting signal as CATV in addition to an ordinary television broadcasting signal, since a broadcasting (carrier) frequency for the CATV is intentionally deviated from the ordinary television broadcasting (carrier) frequency by a fixed frequency, a local oscillating frequency of a tuner needs to be deviated corresponding to the above described deviation when selecting channels for the CATV. More specifically, when the AFT signal detecting circuit detects the above-described fixed deviation of the IF signal frequency, the channel selection control circuit microcomputer makes the determination that the channel for CATV broadcasting is selected. As the foregoing, the PLL frequency synthesizer system television signal receiving apparatus utilizing an AFT signal detecting circuit for channel selection for the CATV broadcasting is disclosed in U.S. Pat. No. 4,426,734, for example.
FIG. 3 is a block diagram showing one example of such a PLL frequency synthesizer system television signal receiving apparatus. The television signal receiving apparatus shown in FIG. 3 is the same as that shown in FIG. 2 except the following points. Namely, intermediate frequency processing circuit 6 includes an AFT signal detecting circuit 6b in addition to video detecting circuit 6a.
Described in more detail, the IF signal supplied from SAW filter 5 is inputted to intermediate frequency processing circuit 6 formed as an IC and first amplified by an intermediate frequency signal amplifying circuit 6c and then applied to a carrier extracting circuit 6d and a video detecting circuit 6a. Carrier extracting circuit 6d having an externally attached detecting coil 6g extracts a carrier signal component of 45.75MHz from the IF signal and applies the same to video detecting circuit 6a and AFT signal detecting circuit 6b. When extracting the carrier signal, a signal radio wave having the carrier frequency of 45.75MHz is intensively radiated from coil 6g.
Video detecting circuit 6a video-detects the carrier signal component applied from carrier extracting circuit 6b and the output of the circuit is supplied from intermediate frequency processing circuit 6 through a LPF 6e. Meanwhile, AFT signal detecting circuit 6b having an externally attached AFT detecting coil 6h generates an AFT signal in response to the carrier signal component applied from carrier extracting circuit 6d. FIG. 4 is a circuit diagram showing the AFT signal detecting circuit 6b in detail.
Referring to FIG. 4, the carrier signal component of 45.75MHz supplied from carrier extracting circuit 6d is inputted to AFT signal detecting circuit 6b through terminals 13 and 14. The AFT signal detecting circuit 6b is basically a double-balanced multiplier comprising transistors Q.sub.1 -Q.sub.6 for multiplying the signal inputted from terminals 13 and 14 by the detection signal from the externally attached AFT detecting coil 6h. The result thereof is outputted from an output terminal 15 and applied to a LPF 6f (FIG. 3). Meanwhile, the reference numeral "12" in FIG. 4 denotes a biasing voltage source.
AFT detecting coil 6h generates the detection signal out of phase by 90.degree. with respect to the carrier signal component of 45.75MHz applied to input terminals 13 and 14, when externally receiving the signal radio wave of 45.75MHz. The above described double-balanced multiplier generates the AFT signal of Vcc/2 and outputs the same through terminal 15 when receiving the signals of 45.75MHz out of phase by 90.degree. from each other. Assuming now that an IF signal for CATV broadcasting having the carrier frequency of 42MHz deviated by 3.75MHz from the above-described carrier frequency of 45.75MHz, for example, is inputted to intermediate frequency processing circuit 6, coil 6g radiates a signal having a frequency of 42MHz to some intensity, less than the intensity for the above-described carrier of 45.75MHz, though. The radiated signal of 42MHz is received by AFT detecting coil 6h. The signal radio wave received by AFT detecting coil 6h is deviated from the above described 45.75MHz, so that the phase deviation between the inputs to the double-balanced multiplier is changed from the above-described 90.degree., corresponding to the deviation. In accordance with the 3.75MHz predetermined deviation width, the double-balanced multiplier generates a voltage which is outputted as an AFT signal through terminal 15. The output is supplied from intermediate frequency processing circuit 6 through LPF 6f of FIG. 3 and fed back to channel selection control circuit 4. The channel selection control circuit 4 receives the AFT signal to carry out the channel selection of changing the frequency-dividing ratio 1/n of frequency dividing circuit 3a until the signal attains a predetermined voltage corresponding to the predetermined deviation of the frequency for the CATV broadcasting.
As the foregoing, AFT signal detecting circuit 6b comprising the double-balanced multiplier generates higher harmonics having a fundamental frequency of an intermediate frequency due to the non-linear characteristic, so that IC 6 is filled with the higher harmonics. The higher harmonics are externally irradiated intensively from coils 6g and 6h to be fed back to antenna 1. When some of such higher harmonics are included in the band of the received channel, various beat interferences such as oblique stripes and wavy patterns appear on the reproduced picture in different manners for each channel.
In order to prevent such beat interference, the entire intermediate frequency processing circuit 6 formed as an IC is conventionally contained in a shield case to cut off the feed-back of the higher harmonics to the antenna, which shielding significantly deteriorates efficiency in manufacturing.
Meanwhile, Japanese Patent Publication No. 63-23715 discloses a frequency synthesizer system television signal receiving apparatus for stopping an AFT operation by cutting off the supply of an AFT signal from an AFT signal detecting circuit to a channel selection control circuit, if required. In this device, however, since the AFT signal detecting circuit itself operates all the time, such problem of radiation of higher harmonics as described above can not be resolved.